Metallic glasses fail by the formation of localized shear bands, which leads to catastrophic failure. Metallic glass specimens that are loaded in a state of plane stress fail on one dominant shear band and show little inelastic behavior. Metallic glass specimens loaded under constrained geometries (plane strain) fail in an elastic-perfectly-plastic manner by the generation of multiple shear bands. Multiple shear bands are observed when the catastrophic instability is avoided via mechanical constraint; e.g., in uniaxial compression, bending, drawing, and under localized indentation. There are a number of models that attempt to describe the formation of shear bands in metallic glasses, and at present these models do not fully describe the experimental observations.
A new class of ductile metal reinforced bulk metallic glass matrix composite materials has been prepared that demonstrate improved mechanical properties. This newly designed engineering material exhibits both improved toughness and a large plastic strain to failure. The new material was designed for use in structural applications (aerospace and automotive, for example), and is also a promising material for application as an armor.
There is provided in practice of this invention, a method for forming a composite metal object comprising ductile crystalline metal particles in an amorphous metal matrix. An alloy is heated above the melting point of the alloy, i.e. above its liquidus temperature. Upon cooling from the high temperature melt, the alloy chemically partitions; i.e., undergoes partial crystallization by nucleation and subsequent growth of a crystalline phase in the remaining liquid. The remaining liquid, after cooling below the glass transition temperature (considered as a solidus) freezes to the amorphous or glassy state, producing a two-phase microstructure containing crystalline particles (or dendrites) in an amorphous metal matrix; i.e., a bulk metallic glass matrix.
This technique may be used to form a composite amorphous metal object having all of its dimensions greater than one millimeter. Such an object comprises an amorphous metal alloy forming a substantially continuous matrix, and a second ductile metal phase embedded in the matrix. For example, the second phase may comprise crystalline metal dendrites having a primary length in the range of from 30 to 150 micrometers and secondary arms having a spacing between adjacent arms in the range of from 1 to 10 micrometers, more commonly in the order of about 6 to 8 micrometers.
In a preferred embodiment the second phase is formed in situ from a molten alloy having an original composition in the range of from 52 to 68 atomic percent zirconium, 3 to 17 atomic percent titanium, 2.5 to 8.5 atomic percent copper, 2 to 7 atomic percent nickel, 5 to 15 atomic percent beryllium, and 3 to 20 atomic percent niobium. Other metals that may be present in lieu of or in addition to niobium are selected from the group consisting of tantalum, tungsten, molybdenum, chromium and vanadium. These elements act to stabilize bcc symmetry crystal structure in Ti- and Zr-based alloys.